Well, 5.7 is out; it is available on the
Russian support site, and 'Open With' is there. But the only options you get for an ePub are Adobe Reader and FBReader. So it does not use extensions.cfg. Bit of a disappointment.
'Open with' is available in the long-tap menu. A short tap still opens the book with the app set as the first in extensions.cfg, so the epubinfo route is still open.